Substrate materials for offset printing plates are provided, either by the user directly or by the producer of precoated printing plates, on one or both sides, with a radiation-sensitive or light-sensitive layer, which is a so-called reproduction layer. With the aid of the sensitive layer an image of an original which is to be printed is produced by a photomechanical method. After exposure and development of the radiation-sensitive layer, the substrate bears the image parts which carry ink during subsequent printing. At the same time the substrate forms the hydrophilic image ground for the lithographic printing process, in the parts which are image-free during subsequent printing, the so-called nonimage parts.
The following requirements are therefore set for a substrate for reproduction layers for the production of offset printing plates:
(i) those parts of the radiation-sensitive layer which have become relatively more soluble after exposure must be capable of being removed readily and completely from the substrate by a development in order to produce the hydrophilic nonimage parts; PA1 (ii) the substrate bared in the nonimage parts must have high affinity to water, i.e., must be strongly hydrophilic, in order to absorb water rapidly and permanently in the lithographic printing process and to have a sufficiently repellant action with respect to the greasy printing ink; and PA1 (iii) the adhesion of the radiation-sensitive layer before exposure or of the printing parts of the layer after exposure must be sufficient. PA1 (a) treating the aluminum oxide layer with an aqueous solution of a pure and crystalline alkali metal silicate, and PA1 (b) rinsing the treated aluminum oxide layer with ion-containing water.
The base material used for such substrates typically is in particular aluminum, which is roughened on the surface by known methods, such as by dry brushing, wet brushing, sand blasting or chemical and/or electro-chemical treatment. To increase the abrasion resistance, the roughened substrate is also usually subjected to an anodizing step to build up a thin oxide layer.
In practice, the substrate materials, in particular anodically oxidized substrate materials based on aluminum, are often subjected, before application of a radiation-sensitive layer, to a further treatment step as described, for example, in EP-B 0 105 170 and EP-B 0 154 201, both of which are incorporated by reference herein in their entireties, to improve the layer adhesion, to increase the hydrophilic character and/or to facilitate development of the radiation-sensitive layers.
EP-B 0 105 170 discloses a process for the after-treatment of aluminum oxide layers with an aqueous alkali metal silicate solution, in which, after the treatment (a) with an aqueous alkali metal silicate solution has been carried out, a treatment (b) with an aqueous solution containing alkaline earth metal salts is additionally carried out. The alkali metal silicate solution is an aqueous solution containing Na.sub.2 SiO.sub.3.5H.sub.2 O. Rinsing is then-effected with distilled water, it also being possible to omit this intermediate cleaning. A treatment in an aqueous solution of an alkaline earth metal nitrate, such as, for example, of a calcium, strontium or barium nitrate, is carried out subsequently or directly after the application of the silicate. The intermediate rinsings with distilled water have a certain effect on the alkali resistance. In particular, alkali resistance is generally better in the case of pores which have not been subjected to intermediate rinsing after the silicate application stage than in the case of pores subjected to intermediate rinsing.
EP-B 0 154 201 describes a process for the after-treatment of aluminum oxide layers in a solution which contains an alkali metal silicate and alkaline earth metal cations. Calcium salts or strontium salts, in particular nitrates or hydroxides, are used as alkaline earth metal salts. The aqueous solution in the after-treatment additionally contains at least one complexing agent for alkaline earth metal ions. The materials are electrochemically roughened in an aqueous solution containing nitric acid. The materials are furthermore anodically oxidized in one stage or in two stages in aqueous solutions containing H.sub.2 SO.sub.4 and/or H.sub.3 PO.sub.4. The after-treatment is carried out electrochemically or by an immersion treatment.
In the case of the substrates which have been treated by the known processes, it is found that the sodium metasilicates frequently used for silicate application, such as, for example, Na.sub.2 SiO.sub.3.5H.sub.2 O, degrade the aluminum oxide very rapidly in an undesirable manner at relatively high pH of the after-treatment solution of 12.2.